Method of pickling metal



Patented Oct. 23, 1934 UNITED STATES PATENT OFFICE METHOD OF PICKLING METAL Laurance F. Van Mater, Lockport, NZY.

No Drawing. Application July30, 1931, Serial No. 554,151

Claims. (01. 204-4) The present invention relates to methods of pickling metallic substances for the removal. of scale therefrom and relates particularly to methods of pickling special alloy steels which are designated as rustless alloys.

In the heat treatment of ferrous alloys the oxidizing atmosphere of the envelope surrounding the metallic object, sheet or castings being heat treated exerts a deleterious oxidizing action upon the metal of the object and produces an oxidized surface or scale thereupon. In order to prepare these shapes or castings for acceptable use by the trade, it has been common practice heretofore to subject such oxidized or scaled castings to the action of certain acid solutions whereby the scale was dissolved or loosened substantially so that it might be readily removed. Pickling baths or solutions for use in conjunction with ordinary steels and ferrous alloys consisted usually of a dilute solution of sulfuric acid to which numerous additional compounds, as for instance common salt, had been added to aid in the attack and removal of the scale.

Additional methods of scale removal depended both upon the aid of chemical action of certain reagents and upon a physical effect whereby the scale was loosened and its removal facilitated. In essence, this method consists of an electrolytic action in acid solutions in which the work 39 piece to be pickled is made the cathode of an electrolytic cell and gaseous hydrogen liberated at this pole. This electrolytic action serves first, to reduce certain oxides of iron which may be present in the scale, and secondly to aid me chanically in loosening the scale and effect the easy removal thereof from the casting.

Under some conditions scale is removed by the mechanical action of a sand blast. Certain of these methods of pickling have been used heretofore with a certain measure of success only when the piece to be pickled comprised a ferrous product or alloy of ordinary composition. Attempts to use the methods of pickling heretofore in use, upon ferrous products of special composition, and especially those which are known as rustless or stainless alloys, have not been attended with amarked degree of success. More especially is this true when these methods of pickling have been applied to the problem of scale removal from certain ferrous alloys containing chromium and/or nickel as alloying constituents. i

It has been found that steels containing chromium and steels containing chromium and nickel in certain proportions, are not amenable to certain of these methods of treatment for scale removal but must be treated by special processes which are ordinarily lengthy and are not particularly advantageous due to the extended time of treatment, the relatively expensive materials used in the process, and the excessive labor charge involved. Thus for the removal of scale from certain types of stainless" alloys, it has been found necessary to sand blast the material and then subsequently treat the sanded material stainless is very coherent and adheres tenaclously to the form or sheet and is variously greenish gray to grayish brown in color. I intend to subject the casting to a treatment'whereby the costeels such as those designated as rustless or B herency and adherence of the scale to the casting is materially decreased, so that the scale may be readily removed either by a subsequent mechanical operation or by a chemical treatment or a combination of mechanical and chemical treat ments.

The instant invention provides a method whereby a coherent and adherent oxide coating on a metal such as a stainless alloy is changed to an oxide or scale which is less coherent and adherent and which lends itself to attack and removal by Q acid pickling reagents or to removal by rubbing or in some cases by a rinsing operation.

In order to facilitate the removal of scale from the special steels containing chromium, nickel,

or chromium and nickel, I so treat the alloy that the oxidized surface condition is accentuated and physical and chemical properties imparted thereto which make the scale easily removable by certain subsequent treatments. In general, therefore, the first step in my process consists in an oxidizing treatment for producing a readily removable scale upon the alloy.

One method by which this particular result may be eflectuated comprises anodic oxidization of the work piece or object to be pickled. The

work piece is therefore made the anode of an electrolytic cell and electric current passed therethrough, certain of the so-called non-metallic ion constituents are liberated at the anode and there results an oxidation of the surface of the anode. That is to say, the broad principle of this step comprises an electrolytic or anodic oxidation of the surface of the work piece for the production of a more readily removable scale formed in a gelatinous, slimy and relatively porous condition which may be readily removed from the casting by a subsequent treatment of suitable nature depending upon the condition of the scale.

In the preferred method'of working my process, I provide an electrolyte, which contains hydroxyl ions and is alkaline in reaction, and immerse the material to be pickled therein. In order to obtain the desired oxidizing effect I connect the work piece to the source of current in such manner that it becomes the anode of the cell while another electrode of relatively indestructible material is provided as a cathode.

The alkalinity of this electrolyte may vary over a wide range, and both strongly and weakly alkaline solutions have been found effective as electrolytes. Ordinarily, somewhat more vigorous oxidizing and scale producing actions are obtained at relatively elevated temperatures, temperatures from 180 F. to 220 F. or in the neighborhood of 200 Fahrenheit. Although the treatment may be eifected at other temperatures, however satisfactory, rapidity of action is obtained at the temperatures specified. Under certain conditions of operation, as for instance with relatively low concentration of caustic soda and high current density, a disagreeable mist is produced over the tanks. However, with high concentration of caustic soda, between 35% and 50% sodium hydroxide, the mist is generally eliminated. Since the higher concentrations of the alkaline solution possess relatively higher boiling points, relatively concentrated solutions are preferred for treating certain alloys. However, as noted above, relatively weak solutions serve eflicaciously as alkaline electrolytes in this process, and solutions containing 8 to 10% of sodium hydroxide may be used.

As alkaline materials I have found eflective the water soluble hydroxides of the alkali metals or of the alkaline earth metals to be very effective but for economy prefer to use hydroxide of sodium. Milder alkalies may be used as for instance certain soluble carbonates of the alkalies, as for instance sodium or potassium carbonate or mixtures of the same, or certain other alkali metal salts which are hydrolyzed in water solution to give an alkaline reaction are eiiicacious. Under certain conditions of operation these mild alkaline reagents may be used, however they are not as eflective and eflicient as caustic soda or potash in concentration ranging up to 50% caustic soda.

The current density which I use to efiect the anodic oxidation may vary over a wide range as I have found that in general the time of treatment necessary to produce a reddish brown scale which is readily removed, decreases as the current density is increased. The color and type of scale differs with the article treated and with the time of treatment, and in some instances may be light brown in color. Prolonged electrolysis produces in general a darker colored scale and one which is substantially less adherent to the work piece than the original scale. Under some conditions of operation, this scale may be re: 1.oved by rubbing or even by subjecting the scaled metal 'to the action of a rinsing bath. With relatively high current density I have been able to produce the desired surface condition upon the work piece in periods of time ranging from fifteen to thirty seconds, while with materially lower current density the time of treatment may be upwards of twenty minutes. With certain types of annealed products or hot rolled material the time may be increased to 2-3 minutes or longer. However, for purposes of economy and rapidity of operation, I prefer to work at a relatively high current density, at relatively elevated temperatures, and for short periods of time and thus produce a scale which lends itself -to subsequent removal by treatment with specialized chemical reagents.

After changing the coherent and adherent scale to a relatively non-coherent and non-adherent scale by my electrolytic method I subject the work piece to a washing or rinsing operation whereby residual alkaline reagent is removed and the alloy is subjected to a further step in the operation for the removal of the relatively non-coherent and non-adherent scale.

This step comprises treating the work piece with a Water solution containing hydrogen ions, that is with an acid pickling reagent which possesses the property of removing the scale with a minimum of chemical reaction and subsequent pitting of the piece to be pickled. The type of acid reagent which I prefer to use on so-called stainless alloys comprises a solution of nitric acid to which a small proportion of a hydrohalogen has been added. As an example I have been able to produce a silvery white surface upon a ferrous allow containing 14.42% chromium, 0.29% silicon, 0.19% nickel, 0.10% carbon, 0.35% manganese, 0.017% phosphorus, 0.043% sulfur by the use of a solution containing 8% nitric acid and less than 1% hydrofluoric acid, the hydrofluoric acid content being, in the example cited, 0.15%.

In general the strength of nitric acid used on stainless alloys may vary. I have found that water solutions containing 5% to 10% nitric acid remove scale, as formed by my method of anodic oxidation, rapidly and effectively.

I have prepared successfully scale-free silvery white surfaces upon so-called stainless or rustless alloys containing 12% to 14% chrr mium and also successfully treated alloys containing 16-18% chromium as well as other classes of alloys designated as stainless alloys, as for instance those alloys containing nickel.

After the acid treatment, the work piece is washed or rinsed, to remove the last trace of acid and is dried in any convenient manner, as for instance by drying with steam.

The particular apparatus or mechanism by means of which I effect the various steps of the process are of conventional type as used in the When scale is to be removed from sheets or strips, the process may be worked as a continuous process by passing the sheet or strip through roller contacts which are connected to an electric circuit in such fashion that when the strip or sheet enters a bath of alkali the sheet or strip is the anode of an electrolytic cell. The rate of passage of the piece through the bath is regulated and determined by the alkaline concentration, and temperature thereof and of the current density at the anode. In most cases the time of contact is less than one minute, although the time factor will vary depending upon the character of the material. alkali concentration,

and anode current density. g

Thesheetorstripthenintoatankof s rinsing water, then to the acid bath where the scale is removed. Traces of acid are eliminated by e of the work piece through a .water rinsing tank and finally into a steam chamber where the piece is dried.

In the case of sheets the operations maybe worked effectively as a batch process where provision is made for indestructible electrodes between which the sheets may be suspended as anodes. After suiilcient electrolytic treailnent in the alkaline bath, the sheets are removed to a rinsing bath and finally the scale removed either 'by mechanical removal or by immersing the sheets in suitable acid and then rinsing and drying.

The above apparatus is suggestive of the means 'by which the process may be worked and any other apparatus suitable may be used. What I claim as my invention is the process of scale removal from metal objects and articles as defined by the scope of the following claims.

What is claimed is:

1. In cleaning a chromium-containing rustless iron and steel product to remove oxide scale therefrom, the art which includes, electrolyzing said product in an alkaline electrolytic bath wherein the product is made the anode, thereby oxidizing the oxide scale thereof and giving 'a scale more susceptible to removal, and then pickling said product in a'nitric acid solution to remove said scale whereby clean. scale-free metal is achieved.

2. In cleaning a heat-treated chromium-containing rustless iron and steel product to remove oxide scale therefrom, the art which includes,

subjecting said productto anodic oxidation in an electrolytic cell having a sodium hydroxide solution electrolyte, thereby oxidizing the oxide scale thereof to a' scale more susceptible to removal. and then washing said product in an acid pickling bath thereby removing said scale and achieving clean bright metal.

s. In cleaning a hot-rolled chromium-containing rustless iron and steel product to remove oxide scale therefrom, the art which includes, electrolyzing said product in an alkaline electrolytic bath wherein the product is made the anode thereby oxidizing the oxide scale thereof, and then pickling said product in a solution of nitric acid containing a small percentage of hydrofluoric acid to remove said scale and give bright scale-free metal.

4. In cleaning a chromium-containing rustless iron and steel product to remove oxide scale therefrom, the art which includes, electrolyzing said product in a 35 per cent to to per cent sodium hydroxide solution wherein the product is made the anode, thereby oxidizing the oxide scale thereof, and then washing said product in a nitric acid solution to remove said scale whereby clean metal is achieved.

5. In cleaning a chromium-containing rustless iron and steel product to remove oxide scale therefrom, the art which includes, subjecting said product to anodic oxidation in an electrolytic cell including an electrolyte of hot sodium hydroxide to transform said oxide scale into a highly oxidized scale. and thereafter washing said product in a 5 per cent to 10 per cent nitric acid solution containing about 1 per cent of hydrofluoric acid to remove said highly oxidized scale and achieve clean metal.

LAURANCE F. VAN MA'I'ER. 

